A silicon based solar cell using monocrystalline silicon, polycrystalline silicon and amorphous silicon has excellent photoelectric conversion efficiency reaching 20% and is put into practical use as a leading technology of the photovoltaic power generation system. However, this silicon based solar cell is high in energy costs on the manufacture of raw materials and is restricted from the viewpoints of price and material supply and the like. On the other hand, in recent years, a dye sensitization type solar cell proposed by Gratzel, et al. gets attention. This has a structure in which an electrolytic liquid lies between a titanium oxide porous electrode having a sensitizing dye supported thereon and a counter electrode, which makes it possible to achieve a large cost reduction from the standpoints of material and manufacturing method and the like.
In this dye sensitization type solar cell, an organic solvent containing an iodine redox pair, such as acetonitrile, ethylene carbonate or the like is used as an electrolytic liquid. For that reason, a lowering of the photoelectric conversion efficiency due to volatilization of the electrolytic liquid, environmental pollution due to leakage of the electrolytic liquid and danger of ignition or the like are pointed out. In order to solve such a problem, a method of using, as an electrolyte, an ionic liquid (ambient-temperature molten salt) which is non-volatile and free from volatilization and ignition is proposed (see, for example, Patent Document 1). A material to be used as the ionic liquid is chiefly a compound having a nitrogen based cation such as imidazolium salts, pyridinium salts, quaternary ammonium salts, pyrrolidinium salts or piperidinium salts (see, for example, Patent Documents 2 to 5). However, in a dye sensitization type solar cell using such an ionic liquid as an electrolyte, there is involved a problem that high viscosity of the ionic liquid affects the diffusion of iodine so that the photoelectric conversion efficiency cannot be made sufficiently high. Furthermore, in even an ionic liquid which is said to be uninflammable, there is no denying of possibility that when exposed at a high temperature, a heat decomposition product is formed and ignited, resulting in causing combustion.
On the other hand, an ionic liquid composed mainly of a phosphorus based quaternary phosphonium cation is also known. It is known that a quaternary phosphonium salt is chemically and thermally stable, and it is also known that the quaternary phosphonium salt has fire retardancy (self-extinguishing properties) in view of the matter that it contains phosphorus. As to the application of the quaternary phosphonium salt to an electrolytic liquid of a dye sensitization type solar cell, an electrolyte composition containing a quaternary ammonium salt and a phosphonium salt composed of an alkyl group or an alkenyl group bonding to a nitrogen atom or a phosphorus atom is described in, for example, Patent Documents 6 and 7. However, all of quaternary phosphonium salt ionic liquids described in these patent documents are high in viscosity, and the problem of a lowering of the photoelectric conversion efficiency has not been solved yet.    [Patent Document 1] JP-A-2002-289267    [Patent Document 2] JP-A-2003-31270    [Patent Document 3] JP-A-2005-85587    [Patent Document 4] JP-A-2005-116367    [Patent Document 5] JP-A-2006-286257    [Patent Document 6] JP-A-2001-35253    [Patent Document 7] WO 02/076924